Hey there!
What I did for mine was take a tiny bit of dirt or potting soil, and rub it onto it. It may seem odd, but it worked for me, so try it! Nothing to lose. Hope this helps!
I tried but they look older , i put the shell in the bleach , it turned black , then i clean it with a special product for silver, the result is ok but i can see the difference between the shell and the coin